1. Field of the Invention
The present invention relates to an abnormality diagnosis apparatus and an engine cooling system having the same.
2. Description of Related Art
In a previously proposed engine cooling system, a bypass fluid line, which conducts coolant and bypasses a radiator, is provided. Furthermore, a thermostat valve, which switches a flow of coolant between the bypass fluid line and a fluid line connected to the radiator, is provided. When a coolant temperature is lower than a predetermined threshold temperature, which indicates an end point of an engine warm-up period, the thermostat valve is controlled to close the fluid line, which leads to the radiator, and opens the bypass fluid line to circulate the coolant through the bypass fluid line.
In place of the thermostat valve, it has been proposed to provide a flow rate control valve, which can adjust a flow rate (bypass flow rate) of coolant that passes through the bypass fluid line. During a warm-up period of the engine, the flow rate control valve switches the flow from the radiator to the bypass fluid line and adjusts the bypass flow rate to control the coolant temperature. However, in such a cooling system having the flow rate control valve, there has not been established a technique for accurately diagnosing existence of abnormality of the flow rate control valve.